Genet Resour Crop Evol (2015) 62:765–794 DOI 10.1007/s10722-014-0194-2 RESEARCH ARTICLE The vulnerability of US apple (Malus) genetic resources Gayle M. Volk • C. Thomas Chao • Jay Norelli • Susan K. Brown • Gennaro Fazio • Cameron Peace • Jim McFerson • Gan-Yuan Zhong • Peter Bretting Received: 20 June 2014 / Accepted: 27 October 2014 / Published online: 13 November 2014 Ó Springer Science+Business Media Dordrecht (outside the USA) 2014 Abstract Apple (Malus 9 domestica Borkh.) is one wide range of biotic and abiotic stress resistances as of the top three US fruit crops in production and value. well as desirable productivity and fruit quality attri- Apple production has high costs for land, labor and butes. However, access to wild materials is limited and inputs, and orchards are a long-term commitment. wild Malus throughout the world is at risk of loss due Production is dominated by only a few apple scion and to human encroachment and changing climatic pat- rootstock cultivars, which increases its susceptibility terns. The USDA-ARS National Plant Germplasm to dynamic external threats. Apple crop wild relatives, System (NPGS) Malus collection, maintained by the including progenitor species Malus sieversii (Ledeb.) Plant Genetic Resources Unit in Geneva, NY, US is M. Roem., Malus orientalis Uglitzk., Malus sylvestris among the largest collections of cultivated apple and (L.) Mill., and Malus prunifolia (Willd.) Borkh., as Malus species in the world. The collection currently well as many other readily hybridized species, have a has 5004 unique accessions in the field and 1603 seed accessions representing M. 9 domestica,33Malus species, and 15 hybrid species. Of the trees in the field, Electronic supplementary material The online version of 3,070 are grafted and are represented by a core this article (doi:10.1007/s10722-014-0194-2) contains sup- collection of 258 individuals. Many wild species plementary material, which is available to authorized users. G. M. Volk (&) C. Peace USDA-ARS National Center for Genetic Resources Department of Horticulture, Washington State University, Preservation, 1111 S. Mason St., Fort Collins, CO 80521, Johnson Hall 39, Pullman, WA 99164, USA USA e-mail: [email protected] J. McFerson Washington Tree Fruit Research Commission, 1719 C. T. Chao Á G. Fazio Á G.-Y. Zhong Springwater Ave., Wenatchee, WA 98801, USA USDA-ARS Plant Genetic Resources Unit, 630 W. North St., Geneva, NY 14456, USA P. Bretting USDA-ARS, George Washington Carver Center, 5601 J. Norelli Sunnyside Ave., Beltsville, MD 20705, USA USDA-ARS Appalachian Fruit Research Laboratory, 2217 Wiltshire Rd., Kearneysville, WV 25430, USA S. K. Brown Department of Horticulture, New York State Agricultural Experiment Station, 630 W. North St., Geneva, NY 14456, USA 123 766 Genet Resour Crop Evol (2015) 62:765–794 accessions are represented as single seedlings (non- In 2011, Vermont was the primary hard cider- grafted). The crop vulnerability status of apple in the producing state in the US. The United Kingdom has US is moderate because although there are a few had the largest world-share of hard cider for decades, breeders developing new commercial cultivars who and cider composes 6 % of the UK alcoholic drink also access Malus species, threats and challenges market. If the US follows the same trend, the hard include new diseases, pests, and changing climate cider industry will become an increasingly large combined with industry needs and consumer demands, market for apples. with a limited number of cultivars in production. Virtually all apple trees planted in commercial orchards were first produced in a nursery by grafting or Keywords Breeding Á Disease resistance Á Genetic budding the scion cultivar onto a rootstock. Rootstock diversity Á Germplasm collections Á Malus Á cultivars are also classified as M. 9 domestica. Apple Wild species rootstocks are produced by layering or stooling propagation beds, which come in full production within 2 years of planting and have a production life span of 15–25 years. US apple rootstock nurseries Introduction to Malus annually produce between 15 and 19 million rootstock liners which are sold or transferred to finished tree Fruit from Malus 9 domestica Borkh. is consumed nurseries and when combined with imported root- fresh, processed, and as juice. According to the Food stocks from Europe result in the annual production of and Agriculture Organization of the United Nations, 17–21 million trees (average cost of $7 per tree) for a apple production is ranked 17th in production value combined farm gate value of $120 million. for agricultural products in both the US and world, Crabapples are typically derived from one or more with an annual crop value of more $31 billion wild species. Crabapples are a steadfast component of worldwide (Online Resource 1). In 2008, there were the landscape industry and are the most widely approximately 7500 US commercial growers (U.S. cultivated small landscape tree in the US and Canada International Trade Commission 2010). Among fruit (Romer et al. 2003). Crabapple trees usually have fruit and nut crops worldwide, apple production value less than 5 cm in diameter. They are valued for their ranked only below grape (Food and Agriculture year-round ornamental attributes such as prolific Organization of the United Nations 2013). flowering, attractive summer foliage, fall fruit color, According to the USDA National Agricultural winter bark coloration and fruit retention for wildlife Statistics Service (2014), the US fresh market apple (Klett and Cox 2008). Crabapple blossom colors range crop was valued at $2.5 billion in 2011, with produc- from white to deep red. Tree architecture can be tion of around 6.3 billion pounds of fruit. Processed weeping, rounded, spreading, upright, vase-shaped, or apples (mostly juice and canned), composed about one pyramidal (Fiala 1994). third of the US apple crop. Processed fruit values were $341 million in 2011 for 3 billion pounds of fruit Dietary contribution (USDA National Agricultural Statistics Service 2014). In the US, the total apple consumption in 2008 was Apples are an important contributor to human health 21 kg per capita, with 14 kg of that in the form of juice as a valuable source of antioxidants and fiber. They are and cider (Economic Research Service 2012). For low-fat, low calorie and low in cholesterol and fresh fruit in 2007, Americans consumed about 7.5 kg sodium. While apples are in ample supply in the US, of fresh apples per capita, while Turkish citizens not all individuals have access to high quality fruit consumed about 32 kg per capita and residents of (Othman et al. 2013). Apples tend to be absent in Italy, New Zealand, Belgium, Germany, and France inner-city ‘‘food deserts’’, yet studies indicate how this consumed more than 14 kg per capita (U.S. Interna- might be changed (Weatherspoon et al. 2013). Long- tional Trade Commission 2010) (Online Resource 2). distance transport of apples is feasible, yet distances The hard cider industry in the US is burgeoning, are a concern for those who consider food miles or with sales of alcoholic cider tripling between 2007 and carbon footprints in their food consumption habits 2012, currently at about $600 million (Furnari 2013). (Blanke and Burdick 2005; Van Passel 2013). 123 Genet Resour Crop Evol (2015) 62:765–794 767 Many studies have demonstrated the beneficial Due to recent declines in honeybee populations, the effects of consuming apples, with benefits ascribed to cost of pollinating orchards has increased (U.S. antioxidants, also known as polyphenols (Espley and International Trade Commission 2010). Pollinizer Martens 2013). The levels of total polyphenols in trees must flower prolifically and at the same time as apple are 5,230–27,240 mg/kg dw in whole apples and the production trees for successful pollination. Fol- 110–970 mg/L in juice (Hyson 2011). Studies suggest lowing successful pollination and fertilization, bloom- that consuming one or more medium-sized apples per thinning methods such as chemical sprays or mechan- day was associated with a reduction in the risk of ical shaking may be applied to achieve durable final cancer and Parkinson’s disease compared to consump- crop loads. tion of fewer apples (Hyson 2011; Kukull 2012). The apple harvest season in the US lasts from Apple consumption has been shown to lead to reduced August to October. Harvested fruit is transported from coronary mortality for both women and men and a the orchard to packers, who distribute fruit to retailers reduced risk of asthma and related symptoms (Hyson and exporters. Fruit are either distributed immediately 2011). Apple consumption may also reduce the risk of to markets or placed into regular or controlled- diabetes (Hyson 2011). Studies have also revealed that atmosphere storage to extend the market life to year- polyphenols in apple peels have beneficial actions on round (Thompson 2010). Fruit are also imported from oxidative stress and inflammation (Denis et al. 2013). the southern hemisphere during late-spring and sum- Apple juice consumption reduced accumulation of mer to help ensure year-round availability of apple reactive oxygen species in brain tissue and attenuated fruit in the northern hemisphere (Canals et al. 2007). cognitive impairment. Domestic and international crop production Apple fruit production Apples are a significant import and export crop for the Cultivated apple trees are perennial and are composed US. In 2011, the US exported more than 820,000 MT of a scion and a rootstock that are vegetatively of fresh apples to Mexico, Canada, India, Taiwan, and propagated and joined together by grafting. Scions Hong Kong. Apple fruit export was valued at $941 are the aerial parts of the tree responsible for million (Foreign Agricultural Service 2013). In addi- photosynthesis and bearing fruit, whereas rootstocks tion, more than 34,000 kL of apple juice was exported support scions by providing anchorage and enhancing to Canada, Japan, and Mexico, with a value of $34.7 uptake of soil nutrients and water.